## Diagnosis of Type 2 Lepra Reaction (ENL) **Key Point:** Type 2 lepra reaction (Erythema Nodosum Leprosum, ENL) is an immune complex-mediated reaction (Type III hypersensitivity) that occurs in lepromatous (LL) and borderline lepromatous (BL) forms of leprosy. It is associated with high bacillary loads and occurs during or after MDT. ### Immunological Basis of Type 2 Reaction ENL is caused by: 1. Antigen-antibody (immune) complex deposition in skin, nerves, and organs 2. Activation of complement cascade 3. Neutrophilic infiltration and vasculitis 4. Occurs in patients with high bacillary loads and antibody production ### Investigations for Type 2 Reaction | Investigation | Finding in ENL | Diagnostic Value | |---|---|---| | **Serum IgM & Circulating Immune Complexes** | Elevated; demonstrates Type III reaction | Most specific | | Skin biopsy | Vasculitis, neutrophilic infiltrate, immune complex deposition | Confirmatory but not specific | | Lepromin test | Negative or weakly positive | Reflects low CMI; not diagnostic | | Slit-skin smear | High AFB count (>4) | Indicates high bacillary load; not diagnostic of reaction | **High-Yield:** ENL occurs in patients with **high bacillary loads** (AFB index >4), whereas Type 1 reactions occur in borderline forms with moderate bacillary loads. This is a key distinguishing feature. **Mnemonic: ENL vs Type 1 Reaction** - **ENL (Type 2):** **E**levated antibodies, **N**eutrophilic infiltrate, **L**epromatous form (high bacilli) - **Type 1:** **T**ype IV hypersensitivity, **B**orderline forms, **T**issue granulomas ### Histopathology Distinction - **Type 1 reaction:** Epithelioid granulomas with caseation (similar to tuberculosis) - **Type 2 reaction (ENL):** Vasculitis with neutrophilic infiltration and immune complex deposition (NOT granulomatous) **Clinical Pearl:** ENL typically presents with systemic symptoms (fever, arthralgia, iritis, neuritis) and occurs in patients with high bacillary loads. It may occur months or years after starting MDT, even after apparent cure. Elevated serum IgM and circulating immune complexes are the most specific investigations. 
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